The present invention relates to MRI (Magnetic Resonance Imaging) methods and apparatus. More specifically, the invention relates to methods of shortening 2D (two-dimensional) excitation pulses for MRI and MRI apparatus using the same.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In general, MRI methods involve pulse sequences that include RF (radio-frequency) pulses and magnetic-field-gradient pulses. M-mode (or “motion” mode) MR is a technique useful for real-time monitoring of the motion of organs such as the diaphragm and heart. M-mode MRI uses 2D excitation pulses to excite a cylindrical or “pencil” region in place of the usual slice excitation, followed by a readout gradient along the pencil axis to produce real-time scrolling 1D images, e.g., of cardiac wall and valve motion.
Insertion of velocity encoding pulses before the readout gradient yields a method for imaging blood flow, e.g., in the heart and major vessels. An ECG-gated (Electrocardiograph-gated) Fourier-velocity-encoding variant of this produces detailed movies of blood velocity distributions over the cardiac cycle, analogous to Doppler M-mode ultrasound. In one non-velocity-encoded application called respiratory navigation, the excited pencil is aligned in the head-foot direction intersecting the diaphragm, and the resulting signal is used to monitor diaphragm position in real time to correct for respiratory motion during imaging. One major problem with the abovementioned techniques is that the time resolution is limited by the extensive duration of the 2D excitation pulses.
The time resolution limited by the extensive duration of the 2D excitation pulses can be remedied in principle with the use of a parallel excitation architecture—multiple transmit elements driven by independent drivers.
However, the parallel excitation approach requires extensive setup and there are currently only a few scanners capable of this. Therefore, there exists a need for reducing the duration of 2D excitation pulses in M-mode MRI without the use of parallel excitation.